How many letters of recommendation should I have for my medical school application?
Students should have at least two letters from science professors and another two from other sources (other courses, coaches, directors at volunteer organizations, etc.).

Who should I choose to write my letters of recommendation?
Your letter writer should know you relatively well.  They should have a knowledge of your academic credentials, but also who you are and why they would recommend you for medical school.  This may be a professor with whom you have worked but may also be their graduate teaching assistant.  The important thing is that the letter writer knows you well.  It is hard in intro classes (which are very large) to get to know the professor, but you can ameliorate this by going to office hours and reaching out.  Also, upper-level classes are usually smaller and allow for more personal attention and interactions.

How soon should I ask for my letter of recommendation?
Request the letter about two months in advance and be clear about the deadline for submission.  Your letter writer likely has a lot on their plate and may not be able to get to it right away.  You are responsible for asking for the letter and for following up with the faculty member.  If you ask them for a letter shortly before it is due, they may feel rushed, shows that you don’t respect their time, shows poor planning, and may result in a less than stellar letter.   However, do send a gentle reminder 2-3 weeks before the deadline, just in case.  Many faculty will appreciate the follow-up.

Should I waive my right to see my letters of recommendation?
Yes.  Otherwise, the admissions committee may wonder why it is important for you to see your letter and be concerned that the letter is not an honest evaluation.